Different materials bring different girth measurements?

I noticed that if I measure with like a piece of paper or something, I don't really get much over 5" girth. When I use a shoe string though, I get 5 1/2". I apply same pressure for both. I'm sure the first is the truer measurement, but how do we know that in those girth measurement statistics didn't use the same thing you're using to measure? This skews things by half an inch, which is significant for girth. It'd be like skewing things an inch for length, I think.

Oh, I've never used an actual fabric measuring thing though. Maybe that would help